Corning Inc. has developed a process known as the fusion process (e.g., overflow downdraw process) to form high quality thin glass sheets that can be used in a variety of devices like flat panel displays. The fusion process is the preferred technique for producing glass sheets used in flat panel displays because the fusion process produces glass sheets whose surfaces have superior flatness and smoothness when compared to glass sheets produced by other methods. The fusion process is described in U.S. Pat. Nos. 3,338,696 and 3,682,609, the contents of which are incorporated herein by reference.
Referring to FIG. 1 (PRIOR ART), there is shown a schematic view of an exemplary glass manufacturing system 100 that implements the fusion process and uses a traditional pull roll apparatus 140 to make a glass sheet 105. The glass manufacturing system 100 includes a melting vessel 110, a fining vessel 115, a mixing vessel 120 (e.g., stir chamber 120), a delivery vessel 125 (e.g., bowl 125), a forming vessel 135 (e.g., isopipe 135), a pull roll apparatus 140 and a traveling anvil machine (TAM) 150.
The melting vessel 110 is where the glass batch materials are introduced as shown by arrow 112 and melted to form molten glass 126. The fining vessel 115 (e.g., finer tube 115) has a high temperature processing area that receives the molten glass 126 (not shown at this point) from the melting vessel 110 and in which bubbles are removed from the molten glass 126. The fining vessel 115 is connected to the mixing vessel 120 (e.g., stir chamber 120) by a finer to stir chamber connecting tube 122. And, the mixing vessel 120 is connected to the delivery vessel 125 by a stir chamber to bowl connecting tube 127. The delivery vessel 125 delivers the molten glass 126 through a downcorner 130 to an inlet 132 and into the forming vessel 135 (e.g., isopipe 135).
The forming vessel 135 which is shown in more detail in FIG. 2 (PRIOR ART) includes an opening 136 that receives the molten glass 126 which flows into a trough 137 and then overflows and runs down two sides 138a and 138b before fusing together at what is known as a root 139. The root 139 is where the two sides 138a and 138b come together and where the two overflow walls of molten glass 126 rejoin (e.g., refuse) before being drawn downward by the traditional pull roll apparatus 140 to form the glass sheet 105. Then, the TAM 150 scores the drawn glass sheet 105 so it can be separated into distinct pieces of glass sheets 155.
As shown in FIG. 2 (PRIOR ART), the exemplary traditional pull roll apparatus 140 includes a first pull roll 142 (having two ends 142a and 142b coated with a compressible refractory roll covering) and a second pull roll 144 (having two ends 144a and 144b coated with a compressible refractory roll covering) which extend across the glass sheet 105 and draw the edges 105a and 105b of the glass sheet 105 (note: FIG. 2 is not to scale). The traditional pull roll apparatus 140 also has a first motor 146 operatively connected to the first pull roll 142 and a second motor 148 operatively connected to the second pull roll 142. Plus, the traditional pull roll apparatus 140 has a device 149 (e.g., computer, programmable logic controller, variable frequency drives) that controls the velocities (revolutions per minute) of motors 146 and 148 to cause the two pull rolls 142 and 144 to draw or stretch the glass sheet 105 (at this point a visco-elastic glass sheet 105) to the desired final thickness. In addition, the traditional pull roll apparatus 140 may include a pair of bare idling rolls 152 and 154 (located below the first and second pull rolls 142 and 144) which have ends 152a, 152b, 154a and 154b that help stabilize and reduce the motion of the glass sheet 105 when the TAM 150 scores the glass sheet 105 and when the scored glass sheet 105 is separated into individual glass sheets 155 (see FIG. 1) (note: for clarity the brackets/supports that hold the pull rolls 142, 144, 152 and 154 have not been shown).
Although the traditional pull roll apparatus 140 works well to draw the glass sheet 105 to the desired thickness there is still a desire to develop a new and improved pulling roll assembly that can draw the glass sheet and at the same time improve the flatness of the glass sheet and also reduce the residual stress within the glass sheet. These particular needs and other needs have been satisfied by the pull roll apparatus and method of the present invention.